FrothNinja
Veteran
I was sitting on my bike when I took this picture this morning, honest.
I'm not sure what it is - it doesn't look deep enough to be a lime kiln, and it's a bit fancy for a railway embankment thingy.
Edit: Google image search says its a lime kiln.
View attachment 682393
Where have you hidden the bike?
Interesting structure. I would be thinking lime kiln but those 'windows' are putting me off. Some pics inside would probably reveal clues - and there should be a feeder at the top. I'm surmising C19 in any case, possibly C18 at a push.